The present invention relates to printing sheets suitable for use as paper currency, securities, admission tickets, etc.
Attempts have been made in recent years to use bills made by printing a resin film as substitutes for conventional paper currency. (The term xe2x80x9cbillxe2x80x9d as used herein includes paper currency and currency made from a resin sheet.) As compared with the conventional paper currency, these bills have the advantage of being superior in durability, water resistance and prevention of counterfeiting.
However, resins generally have the drawbacks of being readily chargeable and difficult to print because of low wettability with ink. Resin films are liable to electrostatically adhere to one another during circulation and therefore difficult to count up or identify by machines, and further have the problem that bills of such film are difficult to count up manually since they are devoid of suitable surface roughness required of bills.
An object of the present invention is to provide printing resin sheets which are free of all the problems including charging and which feel like paper currency.
The present invention provides a printing sheet shaped from a resin containing 5 to 80 wt. % of a fibrous electrically conductive filler less than 100 xcexa9xc2x7cm in volume resistivity, having 3 to 50 xcexcm in average fiber length, 0.01 to 5 xcexcm in average fiber diameter and 3 to 100 in aspect ratio and having a surface coated with an electrically conductive layer containing at least a tin oxide and an antimony oxide.
The present invention further provides a printing sheet comprising a film substrate and a covering layer formed on at least one surface of the substrate and comprising a resin containing the above-mentioned conductive filler.
We have conducted intensive research in view of the foregoing problems and consequently found that a sheet feeling like paper currency and free of any of the problems such as charging can be realized by using a resin having a fibrous material of specific shape incorporated therein. This finding has matured to the present invention.
The fibrous conductive filler for use in the invention comprises a core material, for which various inorganic or organic fibrous materials of specified shape are usable. Examples of such materials usable are potassium tetratitanate fiber, potassium hexatitanate fiber, potassium octatitanate fiber, titania fiber, monoclinic titania fiber, aluminum borate fiber, magnesium borate fiber, alumina fiber, wollastonite, xonotlite, silicon nitride fiber, boron fiber, boron fiber, glass fiber, siliceous fiber, carbon fiber, cellulose fiber, polyester fiber and polyamide fiber. Among these, monoclinic titania fiber and potassium hexatitanate fiber are especially suited for fulfilling the object of the invention since these fibers have the feature of being relatively small in hiding power, therefore permit watermarking, and are further useful for reinforcing the resulting sheet.
The fibrous conductive filler for use in the present invention comprises such a fibrous core material and a conductive layer formed thereon and containing at least a tin oxide and an antimony oxide. The conductive layer containing these oxides is suited to the contemplated use since the layer has high conductivity and is white. Although the method of covering the core material with the conductive layer is not limited specifically, it is desirable, for example, to simultaneously add an alkali and a hydrochloric acid solution of tin oxide and antimony oxide dropwise to a slurry of the core material to effect coprecipitation. While the amount of coating is suitably determined so that the resulting filler has a volume resistivity of less than 100 xcexa9xc2x7cm, preferably 10xe2x88x922 to 10 xcexa9xc2x7cm, usually used for coating are 5 to 100 parts by weight of tin oxide and 0.01 to 10 parts by weight of antimony oxide per 100 parts by weight of the core material. In addition to these oxides, an indium oxide, a cobalt oxide, etc. are also usable suitably. In this case, each or one of the additional oxides is used in an amount of about 0.01 to about 10 parts by weight per 100 parts by weight of the core material.
These methods are disclosed in detail in JP-B-26933/1986, JP-B-4328/1987, JP-B-23221/1995, JP-B-54644/1995, JP-A-170860/1990, etc. Such fillers are commercially available under the brand names of xe2x80x9cDentall WK300,xe2x80x9d and xe2x80x9cDentall WK500xe2x80x9d (both products of OTSUKA KAGAKU KABUSHIKI KAISHA).
The printing resin sheet of the invention can be prepared either by incorporating the fibrous conductive filler into a film substrate itself (claim 1), or by forming a covering layer containing the fibrous conductive filler on one or each of opposite surfaces of a film substrate (claim 2).
When the fibrous conductive filler is to be incorporated into a resin sheet itself, a resin and the fibrous conductive filler are mixed together by dry blending or by melting and kneading, and the mixture is formed into a sheet as by extrusion in a molten state, biaxial orientation, calendering or casting.
The resin to be used is not limited specifically, but various resins are usable which can be made into a film or sheet. Specific examples of resins to be used are polyethylene resin, polypropylene resin and like polyolefin resins, polyethylene terephthalate resin, aramid resin, polyamide resin, polyetheretherketone resin, polyetherimide resin, polyimide resin, etc.
The proportion of the fibrous conductive filler relative to the resin can be suitably determined that the resulting sheet will have surface resistance in the range of 10 to 1010 xcexa9 and retain flexibility without impairment. The filler is used usually in a proportion of about 5 to about 80 wt. %, preferably about 10 to about 40 wt. %, of the combined amount of the resin and the filler.
When a covering layer containing the fibrous conductive filler is to be formed on a surface of a resin sheet (film substrate), a resin composition prepared by admixing the filler with a binder resin is first formed into a sheet, which is then laminated to the film substrate. Alternatively, such a resin composition, which is not cured and wherein the binder resin is a curable resin, is applied to the film substrate surface, and the coating is cured by heating and drying or by irradiation with ultraviolet rays.
Examples of binder resins usable are olefin resin, melamine resin, urea resin, acrylic resin, urethane resin, silicone resin and phenolic resin. The filler is used usually in a proportion of about 5 to about 80 wt. %, preferably about 20 to about 50 wt. %, of the combined amount of the resin and the filler.
In preparing the printing resin sheet of the invention, various methods are usable for the prevention of counterfeiting which include, for example, sandwiching metal tape yarns between the components of the sheet, or incorporating a magnetic material into the resin. Various inks are usable for printing the resin sheet obtained.
Examples of printing methods usable are flexographic printing, gravure printing, offset printing, screen printing and ink-jet printing.